1. Field of the Invention
This invention relates generally to a scroll-type fluid displacement apparatus such as a compressor or pump, and more particularly, to a scroll-type fluid displacement apparatus incorporating a variable displacement device.
2. Background of the Prior Art
Scroll type fluid displacement apparatus are well known in the prior art. For example, U.S. Pat. No. 801,182 to Creux discloses such a device which includes two scrolls each having a circular end plate and a spiroidal or involute spiral element. The scrolls are maintained angularly and radially offset so that both spiral elements interfit to make a plurality of line contacts between their spiral curved surfaces to thereby seal off and define at least one pair of fluid pockets. The relative orbital 5motion of the two scrolls shifts the line contacts along the spiral curved surfaces and, as a result, the volume of the fluid pockets changes. Since the volume of the fluid pockets increases or decreases dependent of the direction of the orbital motion, a scroll type fluid displacement apparatus may be used to compress, expand or pump fluids.
Scroll type fluid displacement apparatus are suitable for use as refrigerant compressors in air conditioners. In this application, thermal control in a room or control of the air conditioner is generally accomplished by intermittent operation of the compressor. Once the temperature in the room has been cooled to a desired temperature, the supplemental refrigerant capacity required of the air conditioner to maintain the room at the desired temperature need not be very large.
When scroll-type compressors are employed in automotive applications, the compressors are typically driven by the engine of the automobile through an electromagnetic clutch. Once the passenger compartment reaches the desired temperature, supplemental cooling is accomplished by intermittent operation of the compressor through the electromagnetic clutch. Thus, the relatively large load which is required to drive the compressor is intermittently applied to the automobile engine. However, capacity control of the air conditioning system by cycling the electromagnetic clutch on and off is not very efficient and stresses the mechanical components of the compressor.
It is, therefore, desirable to provide a scroll type compressor with a displacement or volume adjusting mechanism which controls compressor capacity as occasion demands, thus eliminating the need for intermittent operation of the compressor and the accompanying stress on the driving source and electromagnetic clutch. In a scroll type compressor, adjustment of capacity can be easily accomplished by controlling the volume of the sealed off fluid pockets. Such a capacity adjusting mechanism is disclosed, for example, in Hiraga et al., U.S. Pat. No. 4,468,178 wherein the adjusting mechanism includes a pair of holes formed through the circular end plates of the one of the scrolls. The holes are symmetrically placed so that the wrap of the other scroll simultaneously crosses over the holes. The opening and closing of the holes is controlled by valves.
In the Hiraga capacity adjusting mechanism, when the pair of holes is opened to effect a reduction in compressor capacity, fluid in the outermost fluid pockets is permitted to leak to the suction chamber through the holes. As fluid passes through the holes, there is a corresponding pressure increase in the suction chamber and a pressure loss in the outermost fluid pockets. The fluid bypass through the pair of holes is typically known in the art as a hot gas bypass.
While the hot gas bypass of partially or completely compressed fluid to a region of lower pressure reduces the capacity of the compressor, it is not without a cost. In particular, the heat generated by the fluid compression process is added to the low pressure fluid. Consequently, fluid heating is significantly increased when the bypass circuit introduces high-pressure, high-temperature fluid to the low-pressure, low-temperature fluid at the suction chamber inlet, often to an extent that mechanical failure of the pump or compressor becomes a concern.
In addition to the problems associated with bypassing fluid from high to low pressure areas in prior art scroll compressors, conventional scroll compressors, when declutched, are characterized by a backflow of compressed gas from the discharge chamber. The backflow causes the orbiting scroll member and associated drive shaft to reverse their direction of rotation. This reverse movement often generates objectionable noise or rumble. Additionally, in some situations, such as a blocked condenser fan, it is possible for the discharge pressure to increase sufficiently to stall the drive motor and effect a reverse rotation thereof. As the orbiting scroll rotates in the reverse direction, the discharge pressure will decrease to a point where the motor again is able to overcome the pressure head and rotate the scroll member in the "forward" direction. However, if the discharge pressure increases to a point where the cycle is repeated, the compressor and its associated parts may be damaged.
In addressing the backflow and subsequent counter-rotation problem, some prior art scroll type compressors introduce a one way check valve in the fluid outlet passage. The check valve prevents counter-rotation of the crankshaft immediately upon disengagement of the clutch. The check valve, however, has necessarily led to increased coolant flow resistance and a consequent reduction in coolant output by the compressor during periods of high heat load. In addition, under certain conditions, an objectionable noise is generated by the operation of the check valve.
Thus, some compressors eliminate check valves while preventing reverse rotation of the orbiting scroll during initial declutching of the compressor. For example, Muir, U.S. Pat. No. 4,998,864 employs a one-way clutch on the drive shaft. When the driving input to the compressor is terminated, any tendency for the Muir .varies.864 orbiting scroll to rotate in reverse is immediately prevented by the one-way clutch. Alternatively, Shimoda et al., U.S. Pat. No. 5,006,045 incorporates a sophisticated braking system comprising an electromotor which generates eddy currents in a rotor fixedly secured to the drive shaft. The eddy currents supply a torque to the orbiting scroll which prevents the reverse rotation thereof. In each of these compressors, a separate reverse rotation protection mechanism must be provided. Thus, the cost, and often the size, of the compressor increases.
Moreover, in prior art compressors in which the drive shaft is driven by a pulley which is selectively clutched and declutched from the power source, there exists, if the compressor locked up, the possibility that the belt would shear across the pulley. In such a situation, the torque on the crankshaft suddenly increases many times beyond the normal operating torque, with the result that the power source stalls, the belt shears or the crankshaft drive mechanism fails. Thus, it is desirable to implement an inexpensive and reliable lockup control device in the event that the compressor locks up during operation.
Still further, in prior art compressors whose power is derived from a pulley having a serpentine belt winding therearound, the power is conventionally transmitted to the drive shaft by selectively engaging an electromagnetic clutch. In these conventional compressors, the pulley typically surrounds the electromagnetic coils. Consequently, the size of the pulley is increased to accommodate the electromagnetic coils. It is well known that a larger pulley rotates slower than a smaller pulley for a given linear velocity of the drive belt. Thus, all things being equal, either the power source must rotate at a faster speed or the displacement of the compressor must be increased in order to deliver the same refrigerant output as a pulley with a smaller diameter.
These and other disadvantages are addressed by the compressor according to the preferred embodiment which is described in more particularity below.